Many cytokine receptors exist naturally as both membrane-bound and soluble forms. Whereas the membrane receptors have an obvious role in signal transduction, the putative immunoregulatory role played by the soluble receptors remains unclear. Although natural forms of soluble IL-4R (sIL-4R) are known to be present in the biologic fluids of normal mice, the mechanisms regulating the production of sIL-4R have not been characterized. In this study, we have developed an ELISA that allows the measurement of sIL-4R without interference from endogenous IL-4, and have analyzed the effect of cellular activation and several cytokines on the secretion of sIL-4R by murine splenic cells. Although normal spleen cells in culture produced low, but detectable levels of sIL-4R under basal conditions, stimulation with the T cell-mitogens, Con A or soluble anti-CD3 antibodies, caused a 10- to 40-fold increase in the production of sIL-4R. Stimulation of B lymphocytes with LPS, however, did not result in significant up-regulation of sIL-4R secretion. Moreover, IL-4, but not other cytokines, was also a potent inducer of sIL-4R production by spleen cells, even in the absence of other stimuli. Blocking experiments with an anti-IL-4 antibody, 11B11, demonstrated that the effect of T cell-mitogens is partially mediated by endogenously produced IL-4. Cell depletion experiments suggested that although the effect of T cell-mitogens was dependent on the presence of viable T cells, all major cell types including T cells, B cells, and macrophages, either resting or activated, were able to up-regulate their secretion of sIL-4R in response to IL-4. Unlike many activities of IL-4, the secretion of sIL-4R by IL-4-stimulated splenic cells was not antagonized by IFN-gamma. These results suggest that the production of sIL-4R is regulated by stimuli leading to T cell activation and IL-4 secretion and are consistent with sIL-4R having an important role in the regulation of IL-4 activity in vivo.